L. interrogans is an invasive bacterium.
Direct invasion and multiplication in the blood and tissue is the main virulence determinant of Leptospira. Although direct inva-sion of leptospires may cause tissue damage, it is now observed that the degree of multiorgan tissue injury does not correlate with number of leptospira in infected tissues. Therefore, it is suggested that other factors, such as the endotoxin, hemolysin, and lipase, may also contribute to pathogenicity of Leptospira. However, the exact virulence factors of leptospira are not known. The factors responsible for causing injury to host tissues remain unclear and appear to be a complex interaction of many factors.
The severity of the leptospirosis depends on (a) host immunity, (b) virulence of infecting strain, and (c) the number of infect-ing leptospires. After Leptospira enters the body through various sites, it multiplies in blood and tissues, resulting in leptospirae-mia. Leptospira can spread to any part of the body, but affects primarily the liver and kidney. In the kidney, the bacterium causes interstitial necrosis and tubular necrosis. Subsequently, the bacterium causes tubular damage, hypovolemia due to dehydration, and altered capillary permeability causing renal failure. In the liver, it caus es centrilobular necrosis and hepa-tocellular dysfunction leading to jaundice. Leptospira may also invade skeletal muscle, causing edema and focal necrosis.
Humoral immunity plays an important role in clearance of leptospires from the circulation. Immune reactions of the host to leptospira appears to be responsible for many complica-tions, such as meningitis and other clinical manifestations seen during the later stage of the disease.